Moving to increase its market share and keep Longs Drug Stores Corp. out of the hands of rival CVS Caremark, Walgreens has offered to buy the Walnut Creek-based regional drug chain.

Together, Walgreen Co. of Deerfield, Ill., and Longs would dominate the pharmacy market in the Bay Area, with almost four times as many stores as the next-largest player. Walgreens stores tend to be concentrated in San Francisco, while Longs has a large presence in the East Bay.

Walgreens CEO Jeffrey Rein said in a letter to Longs' board that his company was willing to pay $75 per share in cash for the chain, $3.50 per share more than CVS Caremark Corp. had agreed to pay a month ago. The deal would value Longs Drug at roughly $3 billion, including debt.

"We are confident that after you have considered our proposal you will agree that its terms are substantially more attractive to your stockholders than the CVS transaction," Rein wrote.

In a short statement, Longs' board said it would consider Walgreens' offer. But the company said it continued to recommend that shareholders accept CVS' tender offer to buy their shares. A spokeswoman declined to comment further.

In addition to the Bay Area, Walgreens has a sizable presence in other markets where Longs has some of its 521 stores, including Nevada and Arizona.

Indeed, in order to win antitrust approval, the two companies might need to shed some pharmacies.

Walgreens, which is the nation's largest drugstore chain with 6,443 stores, said it already had enlisted two real estate firms to "address any potential store sales that may be necessary in connection with the potential transaction."

Andrew Wolf of BB&T Capital Markets said that in considering Walgreens' offer, Longs' board would have to "weigh the price offered ... against the risk of the deal not happening."

Wolf said that while the CVS deal had already been approved by antitrust authorities, it might be more difficult for Walgreens to win regulatory approval because there is more overlap in the locations of Longs Drug and Walgreens stores.

Walgreens spokesman Michael Polzin said the deal would allow Walgreens, which has targeted both California, where it has 492 stores, and Hawaii, where it has one, for expansion, to broaden its presence in both markets.

He said the deal would give Walgreens "a much more immediate stronger presence than if we grew solely organically."

For CVS, Longs would give the company - which has no full-service drugstores in Northern California or in Hawaii - an entrance to both those markets.

It is unclear how CVS will respond.

In a statement Monday, CEO Tom Ryan said, "Our offer represents a full and fair price for Longs shares, and we stand firm on our price."

Already, though, investors seem to expect that CVS might increase its bid, or another company might join the fray.

On a down day for Wall Street, Longs Drug's stock jumped 5.7 percent, or $4.09, to close at $75.75, above the price Walgreens said it was willing to pay.